How do I make 'X-Axis Mirror' available in Pose Mode?
If you want to mirror your animation form the left side to the other side during pose mode you just need to select the whole left side then copy the pose with Ctrl+C.

After that press Ctrl+i to go to your right side and finally press Shift+Ctrl+V to paste the the X flipped pose, that way you'll have a perfect mirror of both side